847. They decided to drive the cow away ______it did more damage.

　 　　A. since　　 B. before　 C. after　 D. when　　　　

[　 ]848 Scientists say it may be five or ten years ____it is possible to test this medicine on human patients..

　 　A. since　　 B. before　 C. after　 D. when　　　　

846. ____quite recently, most mothers in Britain did not take paid work outside the home.

　 　A. Before　 B. Until　 C. From　 D. Since　　　

845. Doing your homework is a sure way to improve your test scores, and this is especially true _____it comes to classroom tests.

　 　　A. when　 B. since　 C. before　 D. after　　　

844. I felt somewhat disappointed and was about to leave, ____something occurred which attracted my attention..

　 　A. unless　 B. until　 C. when　 D. while　　　　　　

843. I have kept that portrait _____I can see it every day, as it always reminds me of my university days in London.

　 　A. which　　 B. where　 C. whether　 D. when　　　

842. ______you are leaving tomorrow, we can eat dinner together tonight.

　 　A. For　 B. Since　 C Before　 D. While　　　　　　　

841. Careful surveys have indicated that as many as 50 percent of patients do not take drugs ___directed.

A. like　　 B. so　 C. which　 D. as　　　　　　　　　

840. ____that my head had cleared my brain was also beginning to work much better..

A. For　　 B. Now　 C. Since　 D. On　　　　　　　　

839. I’ll accept my job ____I don’t have to get up early.

A. as long as　　 B. in　 case　　 C. though　　 D. now that　

838. I had just started back for the house to change my clothes, _______I heard voices.

A. as　 B. when　 C. after　 D. while